Nonreciprocal spontaneous parametric process

Mediated by interaction with quantum vacuum fields, a laser field propagatingin a nonlinear optical medium can generate new light fields via spontaneousparametric process. Such process is inherent independent of the propagationdirection of light and reciprocal thus far. In this work, we experimentallydemonstrate a nonreciprocal spontaneous parametric four-wave mixing process insodium atomic vapors with dispersive nonlinearity and further broadband opticalisolation by unidirectionally coupling the probe field to a specific quantumvacuum field in another four-wave mixing process. Thanks to the broad bandwidthof the spontaneous parametric process, we achieve optical isolation with abandwidth larger than 100 GHz for isolation ratio over 25 dB. Spontaneousparametric processes and wave mixing in nonlinear medium has been realized inon-chip photonic circuits on diverse platforms. Therefore, this work paves theway for integrated broadband optical isolations and thus can boost scalabilityand function of photonic chips.